docs: def externalHostname(hosts):
      """Ensure external hostname is routable."""
      hostname = hosts[0][0]
      if hostname in localHostnames and len(hosts) > 1:
          hostname = socket.getfqdn().split(".")[0]
          try:
              socket.getaddrinfo(hostname, None)
          except socket.gaierror:
              raise Exception("\nThe first host (containing a broker) is not"
                              " routable.\nMake sure the address is correct.")
      return hostname